Shrub Encroachment Increases Deep Soil Carbon Stocks and Reduces Carbon Turnover Rates in Alpine Meadows on the Tibetan Plateau

ABSTRACT Background Shrubs have been expanding on the alpine meadows of the Tibetan Plateau (TP). Aim However, the effects of interactions between shrub size and soil depth on soil organic carbon (SOC) remain unclear. Methods In this study, we selected alpine meadows encroached by Lonicera rupicola, Potentilla fruticosa, and Caragana erinacea, establishing sampling sites across three shrub sizes (small, medium, and large) and grass patches. We then measured SOC stocks and estimated SOC turnover rates using the isotopic enrichment factor β at a soil depth of 1 m. Results The results showed that all three typical expanding shrubs increased the SOC stocks, which mainly occurred in the deep soil of 20–100 cm. However, SOC stocks in the 0–10 cm layer tended to decrease with increasing shrub size, whereas the 10–20 cm layer exhibited no significant differences in SOC stocks across different vegetation types. Soils under shrubs also showed significantly higher β values compared to grass soils, particularly under large shrub patches, thus suggesting a slower SOC turnover. A positive correlation was found between β and SOC stock, indicating that the soil carbon (C) sequestration capacity of alpine meadows was enhanced after shrub expansion. Conclusion This study emphasizes the importance of deep SOC in shrub expansion research and provides new insights into the assessment and prediction of C sink functions in alpine meadows in the context of global change.